Focusing screens

There are many types of focusing screen.

 

Most of my focussing screens feature fixed guides and a textured surface - the textured surface makes the viewfinder looks permanently dusty.

 

One of my cameras has a "Micro Prism Split Image" surrounded by various textures in concentric rings - the complexity of the image makes small targets harder to find in the viewfinder.

 

Why does so much engineering go into focussing screens, and what is wrong with simple cross hairs on a smooth surface?

The split image and textures etc are combined in the standard screens to offer the ability or flexibility to focus under most conditions and with most lenses. For example, sometimes speed is more important than precision and vice versa.

Many high end cameras offer specialised screens to allow for the best possible match with lens and subject.

Nikon F3 as an example.

To be honest I find the screens annoying dust traps (electrostatic?), fiddly to clean/replace, and never the right one at the right time.

 

Do any cameras have multiple screens to rotate and select the right screen for the environmental situation?

Do they offer any eye protection?

Is there any harm is just removing them?

Edited February 10, 2021 by G&R

Screens can be avoided if you get a rangefinder camera or a mirrorless digital camera. SLR/DSLR will need a screen to make the image visible to the eye (unless you use live view on the latter).

Is there any harm is just removing them?

Try it. You might then understand why the dust-trapping texture is there.

If you're careful when changing lenses, you shouldn't really encounter much "dust-trapping" even over 5-10 years usage.

The camera lens images by projecting a image of the outside world onto film or digital imaging chip. Most cameras are equipped with some method that allows the photographer to preview this image for purposes of composition and focusing. This projected image is an “aerial” image. An “aerial” image can only be seen if it somehow made visible.

 

Traditionally we view this image by intercepting it using a flat glass surface that has been toughed-up. In other words, one surface is ground using an abrasive. This “ground” glass textured surface yields a viable image that is dim, inverted literally reversed and vignetted. One could substitute tissue paper to preform this task.

 

For most camera applications, the ground glass viewing screen is supplemented using a flat Fresnel lens. This lens consists of concentric circles etched on its surface that form a lens-like effect. The Fresnel reduces the vignette and brightens the image. Because the Fresnel interferes with image acuity, generally it is excluded from the central portion of the viewing screen. In other words, the central area is plain ground glass.

 

To improve the ability of the vies scene to perform and assist as a focusing aid, various additional schemes are employed. Often the center at the center of the view screen is a double prism array that yields a split image when focusing is amiss. This scheme works OK when the lens is not an extreme wide-angle or telephoto. To augment focusing a “microprism” is provided. This system works best at small apertures. Most common is a view screen incorporating several techniques.

 

The view scene can even be clear flat glass. Light rays from the camera lens traverse the clear screen area. A strong magnifier, such as the eyepiece lens of the camera, intercepts these rays. Thus, the camera lens and the eyepiece lens, together works much like a microscope / telescope system. In other words, the clear screen area together with the eyepiece lens preforms to allow precise focusing.

Modern Photography 1969-03

When you had to manually focus, there were lots of alternatives, many designed for specific purposes and even specific lenses.

 

What screen you had often was critical, it was not just a folly on the part of the manufacturer

 

 

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The view scene can even be clear flat glass. Light rays from the camera lens traverse the clear screen area. A strong magnifier, such as the eyepiece lens of the camera, intercepts these rays. Thus, the camera lens and the eyepiece lens, together works much like a microscope / telescope system. In other words, the clear screen area together with the eyepiece lens preforms to allow precise focusing.

The clear screen method needs a mark in the plane of focus, i.e. on the screen to help the eye find the distance to focus on. Without such a mark, the eye can still focus quite a bit in front of and beyond the actual place it should be focussed on, being the desired image plane.

When focussed on that mark you do move your eye from the mark to the subject and back again, repeatedly and rapidly. You will see both in focus, provided the lens is near correct focus, even when they are not in the same plane, i.e. when the lens needs to be adjusted to get the subject in focus on film/sensor.

The thing to do then (it gets complicated) is to move the eye sideways a bit. If the mark on the screen and the part of the subject you want focus to be on move, but not relative to one another, the image of that part of the subject will be in the same plane as the mark, and you're done. So you adjust focus while looking from mark to subject and moving the eye sideways until you see no movement of the mark relative to the subject. When both the image of the bit of the subject and the mark are in the same plane, there will no longer be any parallax between the two. Hence the name parallax focusing for this technique.

 

It works very well, especially when the image is too dark to be used with a scattering ground glass screen. But it is slow and requires some getting used to, some practice.

And you still need that dust collecting clear glass screen to put the mark on. Unless it can be suspended in the viewfinder optics, or is 'projected' (using prisms, similar to how frames and the second image are inserted into rangefinders and/or scopes) into the image the lens forms of the subject. A clear screen with mark(s) instead of a frosted one will be possible in cameras. The other options not.

Yes, I have an enlarger focus aid that works that way. Looking through the view port, you move your head side to side, and see if the image moves relative to a grid. If the focus plane is the same as the grid, the image won't move. I don't know that anyone uses this for a camera view screen.

Without such a mark, the eye can still focus quite a bit in front of and beyond the actual place it should be focussed on, being the desired image plane.

This is the nugget that made the penny drop on my end.

This is the nugget that made the penny drop on my end.

The clear area of the SLR focusing screen works without a grid or mark. This is because focus is achieved when the apex of the rays formed by the camera lens kiss off with the apex of the rays from the eyepiece lens. In the case of the SLR it is fixed i.e. the apex of the rays produced by the eyepiece lens kiss off on the surface (image plane) of the view screen. In most applications, a mark on this surface is a requirement. Additionally the power of the eyepiece lens creates a shallow depth-of-focus.

The SLR eyepiece depth of focus might be less than for the enlarger focus device.

 

But in either case, we might want to focus more accurately than that.

The SLR eyepiece depth of focus might be less than for the enlarger focus device.

 

But in either case, we might want to focus more accurately than that.

I don't think depth of focus is ever small enough to focus without mark. It is not just the coincidence of apices, and how small the depth of focus is, but our flexible eyes also enter into the equation and mess up things, unless provided with an anchor point. Maybe in high magnification microscopy? Still eyepiece settings make a visible difference there too.

 

But anyway: i do not think that clear focusiing screens were available without marks.